Adaptive folding

ABSTRACT

A method and device for folding a metal sheet to a well-defined angle, in which the sheet is deformed in the recess of a die through a punch being moved above the recess and pressing on the sheet, towards the die, until it reaches a well-defined end position. The movement of the punch is regulated as a function of the folding force and of the folding angle. The punch is moved in the direction of the die until it is in an end position Ya corresponding to: Ya=Yt-dYw-dYr in which Yt is a calculated position, dYw is determined through extrapolation of the course of the folding angle as a function of the position of the punch, and dYr is the correction of the position of the punch which is necessary to compensate for the spring-back of the sheet.

BACKGROUND OF THE INVENTION

The present invention relates to a method for folding a metal sheet to awell-defined angle.

In a known method, a metal sheet is deformed in a recess of a diethrough a pressure means being moved above the recess, pressing on thesheet, towards the die until it reaches a well-defined end position.

The punch is then raised again, and the sheet springs back elastically alittle. The difference in angle between the situation in which the sheetis clamped between the punch in its minimal position and the vee-blockand the situation in which the sheet is entirely clear of the tools isknown as the spring-back.

In order to form a fold with a desired angle, the Y-position which thepunch must assume is determined by means of a folding formula or table.Folding formulae and tables only take account of the sheet thickness andthe V-opening. They generally do not take account of material propertiessuch as tensile strength and rolling direction, but the latter alsoinfluence the folding angle obtained. The folding angle actuallyobtained will therefore be only approximately the same as the desiredfolding angle.

After the sheet is clear of the tools, the angle obtained is measured.On the basis of the deviation in angle, a calculation of the correctionnecessary on the Y-position of the punch is made. A fold is made in anew sheet taking into account the correction in Y, and the result ismeasured again. This is iterated until the fold angle obtained lieswithin the desired accuracy limits.

It is important to point out that the folding formula which gives theY-position as a function of the desired angle is only an approximation.For, the angle obtained greatly depends on the material used, thecomposition of the material, the texture of the material, the directionof the fold relative to the rolling direction, and the thickness of thematerial. All these parameters which influence the angle obtained ensurethat, for each fold, the correction must be established experimentallyin order to obtain a fold within the desired accuracy limits.

The angle can also be measured in each case only after the sheet isclear of the tools, which is when the spring-back has been completed. Itgoes without saying that this experimental optimization of the foldingangle is laborious and time-consuming. Moreover, it is always necessaryto make a number of test pieces, which are wasted.

In the case of modern folding presses, the folding formula isincorporated in the control system. The operator thus merely has toenter his experimentally measured angle in the control system. Thecontrol system then automatically calculates the correction on theY-position for the next test piece, but here again the angle is stillmeasured manually, the optimization is still laborious, and worthlesstest pieces have to be made.

This has to be carried out for each fold of a particular workpiece. Onceall folds of a particular workpiece have been optimized, production of aseries of the same pieces can be commenced. It is then assumed that allpieces of the range are so similar that the corrections applied, whichwere optimal for the first piece, are applicable to all pieces. Needlessto say, thickness variations, variations in material properties and adifferent orientation relative to the rolling direction will cause adeviation in the angle obtained.

The method according to the invention is a method by means of which thefolding to the desired folding angle is optimized during the making ofthe fold, i.e. it is an adaptive folding method.

A number of adaptive folding methods are known. Such methods are:

a. Method in which the Force as a Function of the Lowering of the Punchis Measured

Methods which measure the force as a function of the position of thepunch aim to derive a number of material properties and thicknessdeviations therefrom. A mathematical model is then used with such datato forecast up to what position (lowering) the punch must go in order toobtain the desired angle.

The force is measured by a force transducer (for example, strain gauges)which is disposed along the force path. The position is given by thelinear encoders already present on the folding press. Such a method cangive good results only insofar as the model corresponds to the actualproperties of the material and the folding process. This method doesmake it possible to work with standard tools.

b. Method in which the Folding Angle is Measured Contactlessly with aVision System

In the case of this method a 2-dimensional image of the folded sheet isformed through projection. The folding angle is then measured on it. Thepunch is thus driven downwards until a desired measured angle isdetected. A certain correction is calculated for the spring-back.

The vision system also works with standard tools, but it has greatlimitations. It often happens that an edge which has already been foldedimpedes the projection of the fold. In those cases the system cannotwork because of the absence of measurement.

Another problem is that in the case of long sheets the angle on the leftcan differ from that on the right. The projection will give an overallimage which corresponds neither to the folding angle on the left nor tothe folding angle on the right.

c. Method in which the Gap between the Sheet and the Vee-Block isMeasured Contactlessly

In the case of this method, a special vee-block which generates a signalwhich changes as a function of the distance of the sheet in thevee-block from the wall inside the vee-block, is used. It is generally avariant of inductive measuring of a position.

The disadvantage is that the vee-block becomes too broad, with theresult that short folds become impossible. Besides, an expensive specialvee-block of that type would have to be purchased for each V-value.

d. Method in which the Angle of the Sheet is Measured by a Contact Piece

In the case of this method a surface is pressed until it is against thefolded sheet. The angle which the surface forms is measured. This isthen also the angle of the folded sheet. The system exists in twoversions.

In a first version the side walls of the vee-block itself are rotatable,and the angle thereof is measured. However, the vee-block broadens as aresult, so that the same disadvantages as those of the other knownmethods apply.

In a second version the contact piece is on the outside of thevee-block, which again prevents short folding.

SUMMARY OF THE INVENTION

The method according to the invention, i.e. an adaptive folding method,is a method by means of which excellent folding results can be obtained,even if the properties of the sheets vary.

The method according to the invention is a method in which a metal sheetis deformed in a recess of a die through a pressure means being movedabove the recess, pressing on the sheet, towards the die until itreaches a well-defined end position.

Said method is characterized in that the movement of the pressure meansis regulated as a function of the folding force and of the folding anglein order, on the one hand, to obtain a well-defined angle before thespring-back of the sheet and, on the other hand, to compensate for thespring-back of the sheet, so that after the spring-back the desiredangle is obtained.

The pressure means is preferably moved in the direction of the die to anend position which is calculated during the folding process according tothe formula:

    Ya=Yt-dYw-dYr

in which

dYw is the correction of the position of the pressure means which isnecessary for obtaining the well-defined angle before the spring-back ofthe sheet, and which through extrapolation of the course of the foldingangle is determined as a function of the position of the pressure means,and

dYr is the correction of the position of the pressure means which isnecessary to compensate for the spring-back of the sheet. For example,dYr can be calculated as dYr=dDr×(dY/dD), dDr being an empirical valuefor the spring-back, and dY/dD being the differential which isdetermined from the course D(Y) of the folding angle D as a function ofposition (Y) of the pressure means. However, dYr can also be determinedfrom the course of the folding force as a function of the position ofthe pressure means.

In one embodiment the method is characterized in that

a. The pressure means is driven in the direction of position Yt, whichis calculated by means of a theoretical folding formula. While thepressure means is moving towards the die and the sheet is thus beingfolded, the course of the folding angle D as a function of the positionof the pressure means Y is calculated, and dYw is calculated from this.The pressure means stops when it reaches the position Yt. An alternativeis that it stops when the position Yw=Yt-dYw is reached.

b. The pressure means is moved over an arbitrary distance away from thedie, so that a spring-back of the sheet occurs, while the course of thefolding force as a function of the position of the pressure means ismeasured.

c. The pressure means is moved in the direction of the die until it isin the end position, which is calculated during the folding processaccording to the formula:

    Ya=Yt-dYw-dYr

in which

dYw is the correction of the position of the pressure means which isneeded to obtain the well-defined angle before the spring-back of thesheet, and which is determined through extrapolation of the course ofthe folding angle as a function of the position of the pressure means,and

dYr is the correction of the position of the pressure means which isneeded to compensate for the spring-back of the sheet, and which isdetermined from the course of the folding force as a function of theposition of the pressure means, or which is calculated asdYr=dDr×(dY/dD), dDr being an empirical value for the spring-back, anddY/dD being the differential which is determined from the course D(Y) ofthe folding angle D as a function of the position (Y) of the pressuremeans.

The correction dYw is advantageously calculated according to theformula:

    dYw=(Dt-Dw).dY/dD

in which dY/dD is the differential which is determined from the courseof the folding angle as a function of the position of the pressuremeans, while the correction dYr is advantageously calculated accordingto the formula: dYr=Yr-Yt, in which Yr is determined throughextrapolation of the course at a folding force which is equal to zero,and Yt is the position of the pressure means (at a folding force whichis equal to 100%, thus just before spring-back).

The folding force can be measured in a multiplicity of pressure piecesdisposed below the die and/or with the aid of strain gauges.

The half folding angle is preferably measured by determining thecoordinates of one point on the underside of the sheet and bydetermining the gradient of the straight line through said point andtouching the lead curve of the die, and the folding angle is taken asdouble the half folding angle. Said half folding angle is advantageouslymeasured on either side of the fold in the sheet, and the folding angleis taken as the sum of the two half folding angles.

For example, a tracer is placed in a reference position by a means, suchas a stop, provided thereon. Said tracer contains a telescopic elementwhich is placed against the underside of the sheet, as a result of whichthe coordinates of a point on the underside of the sheet are calculatedfrom the position of the telescopic element.

The present invention also relates to a device for folding a sheet,comprising a table on which a die is disposed and a pressure means forfolding a sheet in a recess provided for the purpose.

In one embodiment a multiplicity of pressure pieces is disposed in thetable below the die, strain gauges being fitted in said pressure piecesfor the purpose of measuring the folding force.

In one embodiment the device comprises a tracer (e.g. movably disposed)for measuring the angle, which tracer is provided at one end with atelescopic element and with a means for setting up the tracer in areference position, which telescopic element in this reference positioncan be extended until it is against the underside of a sheet placed onthe die.

The device advantageously also comprises means for recording theextended position of said element and for determining the folding angleof the sheet from said position.

A calibration block can be used to determine the relation between thesignal generated by the tracer and the folding angle.

According to a characteristic of that embodiment, one end of at leastone lever is rotatably fixed to the tracer, while the other end of saidlever is rotatably fixed to a fixed part of the device. The device alsocomprises an immovably fixed actuator, a part of which can be driven sothat it assumes at least two positions. Said part is rotatably fixed toa lever, so that in a first position of said part the tracer assumes thereference position, and in a second position of said part the tracer ispositioned outside the working range of the die.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and details of the invention will emerge from thefollowing description, in which reference is made to the appendeddrawings, in which:

FIG. 1 shows a device for folding a sheet;

FIG. 2 shows a tracer for measuring the angle; and

FIG. 3 shows a graph which indicates the force F and the angle D as afunction of the movement Y of the punch.

DETAILED DESCRIPTION OF THE DRAWING

An embodiment of a method according to the invention will be describedbelow.

In said method a metal sheet 2 is folded to a well-defined angle Dwthrough deformation of the sheet 2 in a recess 31 of a die 3. For saiddeformation a pressure means 1 is moved above the recess 31, pressing onthe sheet 2, towards the die 3 to a well-defined end position Ya.

Using a theoretical folding formula, the position Yt to which thepressure means or the punch 1 must move in order to obtain the desiredfolding angle Dw is calculated.

The punch 1 is driven towards said calculated position Yt, as a resultof which the sheet 2 is folded. During folding of the sheet 2 the courseD(Y) as a function of the position Y or the movement of the punch 1 ismeasured.

The moment the calculated position Yt of the punch 1 is reached, theactual folding angle Dt is measured (i.e. before spring-back). Thedifference between the desired angle Dw and the angle Dt obtained iscalled dDw. The value dYw is calculated as dDw×dY/dD. This is the depthcompensation which would be necessary to obtain the desired angle Dwwithout taking spring-back into account. The force present at thatmoment is called Ft and considered to be equal to 100%.

The punch is now driven upwards until the force has decreased to Fe,which is x% of the force Ft. A position Ye is now reached. The value Yris determined from the regression of the curve F(Y) between Yt and Ye.The value dYr=Yr-Yt is equal to the depth compensation which would beneeded to compensate for the spring-back of the material.

In another embodiment dYr is calculated as:

    dYr=dDr×(dY/dD)

    dYr=Yr-Yt

Through the spring-back, Yr=Yt (1-α), in which α is ± constant for atype of sheet and can be calculated, for example, as follows: ##EQU1##in which Y is the distance between die and punch. dY/dD can easily bedetermined from the course D(Y) as a function of Y.

For example, dY/dD can be determined as follows:

    Yt-Yf/Dt-Df

The punch is now driven to the position Ya=Yt-dYw-dYr. After spring-backthe angle Dw will be obtained.

In that advantageous embodiment each fold is thus made in 3 phases: Thepunch first goes down to position Yt or Yw, then comes up a little (e.g.to position Ye if the punch was moved in the first step to position Yt),and then goes down to the final position.

The force is measured by measuring the deformation somewhere along theforce path. This is carried out by means of strain gauges which arefitted in the pressure pieces 17 of the bending table 16. The signalfrom the strain gauges is a measure of the force which the machineundergoes as a result of the material to be folded.

A force transducer is built into each pressure piece. The force wherethe folding is being carried out can always be measured by said forcetransducer. Even in the case of short, thin sheets requiring only asmall folding force, this ensures that an accurate force signal canstill be obtained.

The folding angle D is measured by direct measurement of the halffolding angle d. This angle is determined as the gradient of thestraight line through a point on the underside of the sheet and touchingthe lead curve of the vee-block. Otherwise d is measured on both sides,and D=d₋₋ left+d₋₋ right; or d is measured only at 1 side, and D=2×d.

The coordinates of the point on the underside of the sheet arecalculated from the position of the pin 4 of a distance sensor 8. Thedistance sensor thus has the function of a tracer, and is called atracer below. The signal from the tracer is a measure of the foldingangle.

The tracer is positioned only when the fold is being made. In order toachieve the movement of the tracer, it is connected in the hinges 5 and7 to two levers 9 and 6. The lower lever 9 is immovably fixed in hinge13 and is driven in hinge 12 by an actuator 14. The actuator is, forexample, a pneumatic cylinder. Lever 6 has hinge 11 as the fixed centreof rotation. The reference position of the tracer is determined throughstop 10, which is immovably connected to the tracer 8, striking againstthe vee-block 3 (see FIG. 2).

When the actuator 14 is not activated, the tracer is withdrawn to belowthe level of the table 15. The working range thus becomes entirely free,and there is nothing at all to impede the positioning of the sheet orthe folding itself (position of the actuator as shown in FIG. 1).

A specific example will now be described with reference to FIG. 3.

Using a theoretical folding formula TP, as a function of the movement ofthe punch Y, the theoretical position Yt or movement of the punch inorder to obtain the desired folding angle Dw, e.g. 90°, is calculated.

During the folding, the actual folding angle D(Y) is measured, inparticular continuously, for various movements Y, as a result of whichthe course of D(Y) is determined.

For the theoretical movement Yt of the punch 1 there is a difference dDwbetween the actual folding angle Dt (114°) and the desired folding angleDw (90°).

The differential dY/dD is measured in the region of Dt for the courseD(Y), as a result of which the correction dYw of the movement of thepunch 1 is calculated, in order to obtain the desired folding angle.##EQU2##

The force Ft for obtaining the actual final folding angle Dt (114°) ismeasured.

During the upward drive of the punch 1 (i.e. the removal of the punch 1from the sheet 2) the force F exerted on the sheet 2 is reduced. Themovement of the punch 1 is controlled until a certain force Fe, which ispreferably lower than 50% of Ft, e.g. 30% of Ft, is obtained. Themovement of the punch Ye until Fe is obtained is measured.

The course F as a function of Y is determined in this way.

The abovementioned course F can be used to calculate the depthcompensation Yr-Yt (=dYr) needed to compensate for the spring-back.

The punch 1 is then driven to the position or movement Ya=Yt-dDw-dYr,with the result that after the removal of the punch 1 and no furtherexertion of a force, the desired folding angle Dw is obtained.

What is claimed is:
 1. A method of folding a metal sheet to awell-defined angle in the recess of a die comprising the steps of:movinga pressure means having a position Y above the recess toward a positionYt which is calculated according to a theoretical formula, the movingpressure means pressing the sheet towards the die and forming a fold inthe sheet having folding angle D, regulating the movement of thepressure means as a function of the folding force exerted on the sheetand as a function of the folding angle D to obtain a well-defined anglebefore the springback of the sheet, the folding force and the foldingangle D being functions of the position Y of the pressure means,measuring the course of the folding angle D of the sheet as a functionof the position Y of the pressure means, said measuring conducted atleast in the region of the position Yt, determining a necessarycorrection dYw to the position of the pressure means in order to obtainthe well-defined angle before the spring-back of the sheet, thedetermination being made through extrapolation of the course of thefolding angle D of the sheet and as a function of position Y of thepressure means, stopping the pressure means when it reaches the positionYt, moving the pressure means over an arbitrary distance away from thedie, so that a spring-back of the sheet occurs, measuring the course ofthe folding force as a function of the position Y of the pressure meansduring its movement away from the die, and moving the pressure means inthe direction of the die until it is in the end position Ya, the valueYa is calculated according to the formula

    Ya=Yt-dYw-dYr

in which dYr is the correction of the position of the pressure meanswhich is needed to compensate for the spring-back of the sheet and whichis determined from the course of the folding force as a function of theposition Y of the pressure means.
 2. A method for folding a metal sheetto a well-defined angle according to claim 1, in which the pressuremeans is moved until it reaches the calculated position Yt-dYw, and inwhichdYr is calculated as dDr×dY/dD, dDr being an empirical value forthe spring-back of the metal sheet, and dY/dD being the differentialwhich is determined from the course of the folding angle D of the metalsheet as a function of position Y of the pressure means.
 3. A method forfolding a metal sheet to a well-defined angle according to claim 2, inwhich the correction dYw is calculated according to the formula:

    dYw=(Dt-Dw)·dY/dD

in which Dw is the desired angle, and dY/dD is a differential determinedfrom the course of the folding angle D of the metal sheet as a functionof the position Y of the pressure means.
 4. A method for folding a metalsheet to a well-defined angle according to claim 1, in which thepressure means is moved until it reaches the calculated position Yt-dYwand the pressure means is thereafter moved away from the die until it isin a position Ye wherein the spring-back of the sheet is not complete,and in which the correction dYr is calculated according to theformula:dYr=Yr-Yt, in which Yr is determined through extrapolation ofthe course of the folding force as a function of the position Y of thepressure means, said determination of Yr made at a folding force whichis equal to zero, and Yt is the position of the pressure means when thepressure means is moved away from the die.
 5. A method for folding ametal sheet to a well-defined angle, according to claim 1, in which thefolding force is measured in one pressure piece disposed below the die.6. A method for folding a metal sheet to a well-defined angle, accordingto claim 1, in which the folding force is measured by strain gauges. 7.A method for folding a metal sheet to a well-defined angle, according toclaim 1, in which a half folding angle d is measured by determining thecoordinates of one point on the underside of the sheet and bydetermining the gradient of the straight line through said point andtouching the lead curve of the die, and in which the folding angle D ofthe metal sheet is taken as double the measured half folding angle d. 8.A method for folding a metal sheet to a well-defined angle, according toclaim 1, in which a half folding angle d is measured on either side ofthe fold in the sheet, and in which the folding angle D is taken as thesum of the two half folding angles d.
 9. A method for folding a metalsheet to a well-defined angle, according to claim 7, in which a traceris placed in a reference position by placing a stop provided thereonagainst a side of the die and placing a telescopic element of saidtracer against the underside of the sheet, and in which the coordinatesare calculated from a point on the underside of the sheet from theposition of the telescopic element.
 10. A method for folding a metalsheet to a well-defined angle, according to claim 7, in which a traceris placed in a reference position by placing a stop provided thereonagainst a side of the die, and in which a telescopic element of saidtracer is placed against the underside of the metal sheet, wherein thecoordinates are calculated from a point on the underside of the metalsheet from the position of the telescopic element, and in which thetracer is removed from the vicinity of the die when the folding angle Ddoes not have to be measured.
 11. A method for folding a metal sheet toa well-defined angle, according to claim 8, in which a tracer is placedin a reference position by placing a stop provided thereon against aside of the die and a telescopic element of said tracer is placedagainst the underside of the sheet wherein the coordinates arecalculated from a point on the underside of the sheet from the positionof the telescopic element.
 12. A method for folding a metal sheet to awell-defined angle, according to claim 8, in which a tracer is placed ina reference position by placing a stop provided thereon against a sideof the die, in which a telescopic element of said tracer is placedagainst the underside of the sheet, in which the coordinates arecalculated from a point on the underside of the sheet from the positionof the telescopic element, and in which a tracer is removed from thevicinity of the die when the folding angle does not have to be measured.13. A device for folding a sheet, comprising a table on which a die isdisposed, and a pressure means for folding a sheet in the recess of thedie, in which a multiplicity of pressure pieces are disposed in thetable below the die, said pressure pieces containing gauges therein formeasuring the folding force, and in which the device contains means forcalculating the end position of the pressure means during a foldingmethod in which the sheet is deformed in the recess of a die, the methodcomprising the steps of:moving a pressure means having a position Yabove the recess toward a position Yt which is calculated according to atheoretical formula, the moving pressure means pressing the sheettowards the die forming a fold in the sheet having folding angle D,regulating the movement of the pressure means as a function of thefolding force exerted on the sheet and as a function of the foldingangle D to obtain a well-defined angle before the springback of thesheet, said folding force and said folding angle D being functions ofthe position Y of the pressure means, measuring the course of thefolding angle D of the metal sheet as a function of the position Y ofthe pressure means, said measuring conducted at least in the region ofthe position Yt, determining a necessary correction dYw to the positionof the pressure means in order to obtain the well-defined angle beforethe spring-back of the sheet, the determination being made throughextrapolation of the course of the folding angle D of the metal sheet asa function of position Y of the pressure means, stopping the pressuremeans when it reaches the position Yt, moving the pressure means over anarbitrary distance away from the die, so that a spring-back of the sheetoccurs, measuring the course of the folding force as a function of theposition Y of the pressure means during its movement away from the die,moving the pressure means in the direction of the die until it is in theend position Ya, the value Ya is calculated according to the formula

    Ya=Yt-dYw-dYr

in which dYr is the correction of the position of the pressure meanswhich is needed to compensate for the spring-back of the sheet and whichis determined from the course of the folding force as a function of theposition Y of the pressure means.
 14. A device for folding a sheetcomprising:a table on which a die is disposed, pressure means forfolding a sheet in a recess on the die, a tracer provided at one endwith a telescopic element, wherein the tracer is movable, means fordisposing the tracer in a reference position, wherein the telescopicelement in this reference position can be extended until it is againstthe underside of a sheet placed on the die, means for recording theextended position of the telescopic element and for determining fromsaid position the folding angle D of the sheet, at least one leverhaving two ends, one end of said lever rotatably fixed to the tracer andthe other end of the lever rotatable fixed to a fixed part of thedevice, and an immovably fixed actuator, a part of the actuator drivenso that it may assume at least two positions the driven part of theactuator rotatably fixed to a lever, so that the tracer assumes thereference position in the first position of the driven part of theactuator and the tracer is positioned outside the working range of thedie in the second position of the driven part of the actuator.
 15. Adevice for folding a sheet according to claim 14, in which the tracer ispositioned below the level of the table in the second position of saidpart of the actuator.
 16. A device for folding a sheet according toclaim 14, in which the actuator is a pneumatic cylinder, wherein thepiston rod of the pneumatic cylinder is rotatably fixed to a lever.